Process for electrodepositing metallic coatings



United States Patent No Drawing. Filed Dec. 8, 1960, Ser. No. 74,482 3Claims. (Cl. 204-38) The present invention concerns the electrolyticcoating of metals, especially of ferrous metals, for the purpose ofprotection or decoration.

It has especially in view the improvement of this coating and theperfection of the manner in which it has been operated up till now bycombining in it a stage of preliminary phosphatization.

The customary practice at present is to phosphatize the metals in orderto provide them with a protection layer which in turn is covered with afinishing coat of paint or enamel.

According to the invention it has been discovered that contrary to theaccepted notion according to which these phosphate layers areelectrically isolating, it is possible, by an appropriate choice of thephosphatizing agents and the operating conditions, to use the phosphatecoats thus formed subsequently as a layer of anchorage forelectrolytically deposited metals.

Thus, according to the invention the metals, and especially the ferrousmetals, which are phosphatized by means of diacid phosphates of heavymetals with a layer weight between 0.2 and -g./m. have proved to beparticularly suitable to receive subsequently an electrolytic deposit ofanother metal.

The object of the invention thus is a process of electrolytic coating ofmetals, especially of ferrous metals, which is particularly remarkablebecause it consists of phosphatizing these metals with an aqueoussolution of diacid phosphate of heavy metal under such conditions that aweight of coating between 0.2 and 10 g./n1. is obtained and proceedingthen by the manner which is usual for the electrolytic coating of metalsthat are thus phospha tized.

The advantages of the present process compared to pre- Vious processesof electrolytic coating are considerable, as will be seen in thefollowing, as far as the adherence of the electrolytic coating and theimprovement of the corrosion resistance of this coating is concerned.

The characteristics 'of the process according to the invention will nowbe examined in more detail.

The diacid phosphates of heavy metals that can be used for the stage ofphosphatization are above all the diacid phosphate of Zinc and thediacid phosphate of manganese of which the first is brighterelectrolytic coatings.

These phosphates in aqueous solution can be put into operation accordingto the usual methods of phosphatization by immersion or by spray oratomization at any temperature between the temperature of theenvironment and the boiling temperature of the solution.

In order to obtain the desired Weight of coating one can adjust theperiod of contact of the metals with the solution and the oxidizingaccelerating agent which is added to the latter. In this respect andalthough all oxidizing agents classically used as accelerators givesatisfactory results it is preferred to use the nitrates or chloratesand their mixtures, and with them present in the solution the resultsobtained are particularly good.

normal current density, which is electrodepositing solution.

3,133,005 Patented lVlay 12, 1964 The pieces thus phosphatizedaresubsequently rinsed in water, and thereafter in a solution whose pHcorrespondsto that of the solution to be used to form the electrolyticcoating or deposition. The surfaces are then immersed in this solutionwith the current on so as to avoid any redissolution of the phosphatedeposit in the actual The electroplating solutions may be the classicalalkaline or acid electroplating solutions used under normal workingconditions, except that initially one operates under a higher currentdensity, e,'g., up to 10 times the then reducedto normal after a fewseconds.

preferred because it permits The metals that can be deposited on thephosphatized pieces are all those that are anodic with regard to thebase metal or which are made anodic with regard to the latter through anincrease of its potential by the phosphate layer.

On iron pieces one can thus deposit electrolytically zinc, nickel (mator bright), chromium, copper, brass, tin, and cadmium which is normallynot anodic with regard to the iron.

In order to illustrate the invention in greater detail somerepresentative examples for carrying out the present process for coatingpieces of ferrous metal or of alloys of zinc or aluminum are set forthbelow.

It is to be understood, however, that the compositions given areillustrative only and the operating conditions are specific for thegiven solutions-only. Other'oxidizing agents, concentrations .andoperating conditions can be used to produce the desired 0.2-10 grams persquare meter coating and, once it is rinsed in a solution having theacidity of the contemplated electroplating solution under a similarcurrent density condition, the phosphate coated surface is prepared toreceive an electrodep'osit.

Example I G./liter Diacid zinc phosphate 18 Zinc nitrate s 7 4Phosphoric acid .24.. 3 Sodium chlorate Y r 7.5

The characteristics of thisrbath are as follows: i Free acidity points 4to 4.8 Total acidity do 27 to 30 Temperature C 65 to Time of treatmentper immersion minutes 5 to 10 The weight of the-layer obtained-is from1.5 to 8.g./m.

Example II V i i v G./liter Diacid zinc phosphate 7.5 Zinc nitrate 10Phosphoric acid 3.4

The characteristics of this bath are as follows:

Total acidity points 20 Temperature C to G./liter Diacid manganesephosphate 48 Manganese nitrate 6 Phosphoric acid 7 3 The characteristicsof this bath are as follows:

Free acidity points 9 to 11 Total acidity do 60 Temperature C 95 Periodof immersion minutes 15 The weight of the layer obtained is from 2 to 10g./m.

The weight of the layer obtained is from 1 to 6 g./m.

Aiter phosphatiziation in any one of the aforementioned baths, thepieces are rinsed in water, then with a diluted solution of sodiumcyanide (it the bath of electrolysis is alkaline) or of hydrochloricacid (it the bath of electrolysis is acid). The quantity of the sodiumcyanide or hydrochloric acid used in the aqueous solution was controlledso as to adjust the pH of the aqueous solution to a value approximatingthe pH of the subsequent electroplating solution. The aqueous solutioncontaining the sodium cyanide or hydrochloric acid at the appropriate pHwas contacted with the phosphated surface of the workpiece electricallycharged as a cathode under a current density con -'tion-correspondingsubstantially to that to be employed in the subsequent electroplatingstep.

These pieces are subsequently immersed under current in any one of thefollowing baths of electrolytic deposit depending upon the coating thatone wishes to obtain.

Example V-Cadmium- Bath l Example VIII-Brass Bath Copper cyanideg./liter Zinc cyanide do 10 Sodium cyanide do 50 Sodium carbonate do 30Density of normal current I amp./=drn. 0.3 Temperature C 20 to 30Tension ..v 2

Variable period of treatment: Fixed at 20 minutes for the tests.

Example IXTinning Bath Sodium stannate g./ liter 80 Caustic soda do 13Density of normal current "amp/din? 1 Tension v 4 to 6 Temperature C i p75 Variable time: fixed at 15 minutes for the tests.

Example X-Nickeling Bath Nickel sulfate -g./liter 250 Nickel chloride do40 Boric acid do 40 Density of normal current a1np./dm. 2 Tension vTemperature C to Variable time: Fixed at 10 minutes for the tests.

The tests described in these examples support the observation that inall the cases the adherence of the electrolytic coating is excellent.However, the deposits made on zinc phosphate according to Examples I, IIand IV are brighter than on manganese phosphate according to Example HI.7

Moreover, these tests clearly show the following advantages of thepresent process as compared to previous Double cadmium and sodiumcyanide g./liter 40 Sodium cyanide do 30 Caustic soda do 10 Usualbrightening agent do 3 Density of normal current ..a.mp./dm. 1 to 1.5Tension v 1 to 3 Temperature of the environment. 7 Variable time: Fixedat 20 minutes for the tests.

Example VI-Zinc Bath Zinc cyanide g./liter ,60 Sodium cyanide do 42 Soda..do.. Usual brightening agent -cm./ liter..- 20 Density of normalcurrent .amp./dm. 3 Tension v 3 to 4 Temperature otthe environment.Variabletime: Fixed at 20 minutes for the tests.

. Example .VIl..-Copper Bath v Copper cyanide g./liter 30 Sodium cyanidedo 37.5 Carbonate do 25 Caustic soda do 10 Rochelle salt do 45 Densityof normal current amp./dm. 2to 3 Temperature C 50 to 55 processes ofelectrolytic coating. The objects w ch are phosphatiz/ed and coatedelectrolytically according to the.

invention show a resistance to corrosion which is much greater than thatof objects which are coated simply electrolytically. Thus the pieceswhich were phosphatized according to any one of the Examples I to IV andcadmium-plated according to Example V and on which an initial point ofcorrosion was marked have resisted 288 hours in an industrial acidatmosphere as compared to 104 hours for pieces which were onlycadmium-plated according to Example V and provided with an initial pointof corrosion.

Besides, it should be emphasized that the preliminary phosphatization ofthe pieces which are to be coated electrolytically is actually not anadditional operation, for it replaces advantageously the electrolyticdegreasing and the anodic attack in acid environment normally used.Finally, the phosphate layer which is deposited according to theinvention favors a better dispersion of the lines of the current andhence a better penetration of the electrolytic deposits. (The needleeffect being distributed over the whole surface of the pieces.)

What is claimed is:

l. A process tor forming electrodeposited coatings on metal surfaceswhich comprises the steps of contacting said metal surface with anaqueous acidic phosphate coating solution selected, from the groupconsisting of zinc and manganese phosphate solutions to form thereon aphosphate coating having a weight in the rangeot 0.2-10 grams per squaremeter, thereafter contacting said phosphate coated surface as a cathodewith an aqueous solution containing a soluble compound selected from thegroup consisting of hydrochloric acid and sodium cyanide in an amount toprovide a pH of a value herein.- a-fiter defined and for a period oftime sufficient to effect stabilization of said phosphate-coatedsurface, thereafter electroplating a metallic, coating directly on saidphos: phate coating, said aqueous solution having been adjusted3,133,005 5 6 to the approximate pH of the electroplating solution andReferences Cited in the file of this patent said aqueous solution beingapplied under approximately UNITED STATES PATENTS similar currentdensity conditions as employed in said electroplating step. 2,132,438Rflmig 00L 1938 2. A process in accordance with claim 1 wherein said 5FOREIGN PATENTS phosphate coated metal is electroplated with a metal345119v F i 8 19 selected firom the group consisting of zinc, nickel,chrorance May 39 mium, copper, brass, tin and cadmium. OTHER REFERENCES3. A process in accordance with claim 1 wherein said L0 Presti: MetalFinishing, October, 1942, pages 533- phosphate coating comprises a zincphosphate coating. 10 536.

1. A PROCESS FOR FORMING ELECTRODEPOSITED COATINGS ON METAL SURFACESWHICH COMPRISES THE STEPS OF CONTACTING SAID METAL SURFACE WITH ANAQUEOUS ACIDIC PHOSPHATE COATING SOLUTION SELECTED FROM THE GROUPCONSISTING OF ZINC AND MANGANESE PHOSPHATE SOLUTIONS TO FORM THEREON APHOSPHATE COATING HAVING A WEIGHT IN THE RANGE OF 0.2-10 GRAMS PERSQUARE METER, THEREAFTER CONTACTING SAID PHOSPHATE COATED SURFACE AS ACATHODE WITH AN AQUEOUS SOLUTION CONTAINING A SOLUBLE COMPOUND SELECTEDFROM THE GROUP CONSISTING OF HYDROCHLORIC ACID AND SODIUM CYANIDE IN ANAMOUNT TO PROVIDE A PH OF A VALUE HEREINAFTER DEFINED AND FOR A PERIODOF TIME SUFFICIENT TO EFFECT STABILIZATION OF SAID PHOSPHATE-COATEDSURFACE, THEREAFTER ELECTROPLATING A METALLIC COATING DIRECTLY ON SAIDPHOSPHATE COATING, SAID AQUEOUS SOLUTION HAVING BEEN ADJUSTED TO THEAPPROXIMATE PH OF THE ELECTROPLATING SOLUTION AND SAID AQUEOUS SOLUTIONBEING APPLIED UNDER APPROXIMATELY SIMILAR CURRENT DENSITY CONDITIONS ASEMPLOYED IN SAID ELECTROPLATING STEP.